This proposal is focused on the roles of transcriptional coactivator protein p300 in mediating the relief of YY1 transcriptional repression by adenovirus E1A. Two specific aims are proposed. First, the biochemical mechanisms underlying the functional interactions among YY1, p300 and E1A will be determined. Both in vitro and vivo protein-binding assays will be performed to test whether E1A interacts with YY1/p300 complex to form a tripartite complex. Second, the mechanisms that p300 relays the activation potential from E1A's binding to YY1/p300 complex to basal transcriptional machinery will be investigated. A combined approach of mutational analysis, protein-binding assays and in vivo functional analysis will be applied to identify the component(s) in basal transcriptional apparatus targeted by the protein 300. Negative regulation of gene expression at the transcriptional level is a pivotal control mechanism in cell growth and differentiation. Disrupting such crucial regulators function may result in abnormal growth and tumorigenesis. It is expected that the proposed studies will provide insight into the long-term goal of these studies: to understand the mechanisms of mammalian transcriptional repression and its alteration by viral oncoproteins.